Level sensors for determining the level of a liquid in a container, for example, central heating oil in a central heating oil tank, are known. The most basic of such level sensors comprise a sight glass in which the level of the liquid varies in proportion to the level of the liquid in the tank. More sophisticated level sensors include float operated needle gauges in which a float which floats on the surface of the liquid is mechanically linked to a needle gauge. Such level sensors suffer from many disadvantages. For example, the sight glass type sensors are prone to discoloration, thus rendering them substantially opaque and preventing sight of the liquid level, other disadvantages of such sight glass type sensors are that they are prone to leakage and are relatively expensive to fit. The float type devices regularly suffer from mechanical failure and are relatively expensive to produce.
Efforts to provide level sensors which overcome the problems of known sensors have been made, and one such level sensor comprises an ultrasonic sensor which transmits an ultrasonic signal towards the surface of the liquid, and a receiver for receiving an echo of the transmitted signal reflected from the surface of the liquid. Control circuitry, which typically comprises a microprocessor, times the time period between transmission of an ultrasonic signal and reception of the reflected signal, and a determination of the level of the surface of the liquid relative to the ultrasonic sensor is made from the time period between transmission and reception of the ultrasonic signal. However, such ultrasonic level sensors suffer from a disadvantage in that they are prone to providing spurious readings of the level when the tank is relatively full. This is due to the fact that the echo of the transmitted signal reflected from the surface of the liquid may be reflected while the signal is still being transmitted, and thus cannot be detected. In such cases, the ultrasonic level sensor would give an inaccurate reading when the tank is full.
A further disadvantage of the majority of level sensors known heretofore is that the reading of the level of the liquid in the tank is provided on the sensor unit itself, thereby requiring a visit to the tank in order to determine the level of oil in the tank. Attempts to provide apparatus which permits remote monitoring of oil level in a tank have been made. Such apparatus comprise a monitoring unit which is located on the tank for monitoring the oil level in the tank, and a remotely located unit which provides an indication of the oil level in the tank. The remote unit may be located within a house, building or the like. A radio transmitter is located in the monitoring unit for transmitting a signal indicative of the monitored level of the oil in the tank, and a radio receiver located in the remote unit for receiving the transmitted signal. The remote unit in general comprises a visual display unit which displays the oil level. A microprocessor, in general, is provided in the remote unit for decoding the received transmitted signal and for operating the visual display unit for displaying the oil level. However, such apparatus suffer from a number of disadvantages, one in particular of which is that a manufacturer must match the monitoring unit and the remote unit of a pair of such units prior to dispatch, since each pair of units must have a unique address. Otherwise, a signal from any one monitoring unit could be received by the remote unit of other monitoring units. Thus, if a number of closely located houses with central heating oil tanks were provided with such apparatus, there is no guarantee that the reading provided by one remote unit would be the reading from the associated monitoring unit. The requirement of matching such pairs of units imposes considerable additional operation steps in the manufacturing of such units, and additionally, unless a very high level of inspection is operated by a manufacturer, the risk of monitoring and remote units being mixed up is relatively high.
A further disadvantage of such ultrasonic sensing units is that in general, they require an operating voltage of the order of 5 to 10 volts. In general, the monitoring units are battery powered, and in order to provide the voltages required, the batteries, in general, are alkaline batteries, which have a high energy content, and also a high self discharge rate. Such batteries require frequent replacement.
There is therefore a need for apparatus for indicating the level of a liquid or other flowable constituent in a container which overcomes at least some of the above stated problems, and there is also a need for apparatus for indicating the state of a characteristic of a constituent in a container which also overcomes at least some of the above stated problems.
The present invention is directed towards providing such an apparatus.